This invention pertains to couplers usable within drilling motors. More specifically it pertains to couplers for use down hole in well drilling to couple the power producing part of the drilling motor to the output shaft. The coupler can be used within directional drilling turbodrills or electrodrills, and it is especially beneficial in positive displacement mud powered drilling motors.
In recent years, down hole motor failures have often been caused by the flexible connectors intrinsic to the motors. Motors used in well drilling are necessarily slender in order to move down the well bore. The slender motor can be quite long and can develop substantial torque. The torque may overload internal shafts if the shafts are flexible. A drilling motor for directional drilling usually has a flexible shaft, either to negotiate a bent motor body or to accept power from progressing cavity motor rotors. Progressive cavity motor rotors do not rotate around a stable centerline, they orbit the general centerline and are connected to the stable output shaft by flexible couplers.
Efforts to use ordinary couplings such as the universal joints found on truck drive shafts have met with little success because of the lubrication problem inherent in mud filled motors. Couplers of many forms have been used with reasonable success, but they have been the weak link in the system. Efforts to lubricate the couplings have taken many forms with limited success. Usually, the life of the sealed coupling have been only slightly longer than the life of the lubricant sealing system.
On the progressing cavity drilling motor, the coupling shaft is in compression while drilling proceeds but the shaft can be in tension when the drilling motor is being lowered into the well. The tension load is not great but it does place demands upon the design.
The compression load is not a major problem and simple ball elements have been used to carry the compressive load and keep the flexure point, or points, of the connector stable.
Sealing means to contain lubricant have evolved through many forms. A flexible elastomer sleeve clamped peripherally above and below the coupling showed promise but they too often get punctured and cause early failures in the down hole drilling assembly. The cost of tripping the drill string to replace a component is very high and demands for reliability have reflected the cost.
There is a dire need to avoid the vulnerability of elastomer sleeves, yet provide a coupling that is of such configuration that it has as much expectable run time in mud as the currently used mud bathed systems. By that process, the prospect of long duration run times down hole is not offset by the prospect of sudden failure when the lubricant seals do fail.
The couplers disclosed have stable flexure points and, individually, may serve connected shafts with rotational axes that cross. To connect shafts with rotational axes that do not cross, two couplers are used and separated by a connecting member having a selected length. The connecting member can serve as a housing for lubricant for either or both couplers. The connecting member can also serve as part of either or both couplers.
The preferred coupling has facing half-couplings, each half-coupling having axially extending jaws peripherally spaced such that the opposing half-couplings can be axially moved together until the jaws interdigitate.
A flexible coupling usable down hole on motors needs stable axes of rotation. Spherical surface pairs, convex and concave in sliding relationship, force flexure to take place at the center of development of the spherical surfaces. One version of the coupling employs a ball, mounted on one half-coupling in rubbing contact with a mating concave surface mounted on the opposing half-coupling. Another version has mating spherical surfaces in broken patches, convex at the end of the jaws on one half-coupling mating with concave patches of spherical surfaces at the place where the end of the convex jaw surfaces rub on the opposing half coupling. In any case, the point of flexure, where the coupled rotational axes cross, is at the center of development of the mating spherical surfaces.
The flexure point being established, the sealing arrangement to keep lubricant inside and mud outside involves a sleeve rigidly attached to one coupling member with a sealing ring situated on the extended portion of the sleeve to extend around the periphery of the mating coupling in a transverse plane containing the point of flexure. The sealing ring then experiences little radial displacement when the coupling flexes. The flexing is normally limited to less than five degrees, and usually about three degrees. With limited flexure, the surfaces on the sleeve that confines the sealing ring needs to clear the peripheral surfaces rubbed by the seal only a small amount. The needed clearances are compatible with the clearances normally required to confine the seal in the presence of the limited pressure differences across the sealing ring. The peripherally extending sealing element mounted in the sleeve sealingly secured on one half-coupling slides, or rubs, on a peripherally extending surface of the other half-coupling to form a fluid tight closure.
The occasional tension tending to separate the jaw coupling is contained by an interfering ring, or flange, on the jaw coupling portion not attached to the sleeve. The interfering ring is opposed by an inwardly extending flange within the sleeve near, or part of, the seal confining, peripherally extending, structure on the sleeve. Very little rotation of the coupling occurs when the coupling is in tension. The motor is not normally delivering power when the coupling is in tension.
It is normally the nature of the motor to provide compressive loads on the coupling when the motor is delivering power. The compressive load transmitted through the coupling causes the cooperating spherical surfaces to stabilize the flexure point of the coupling.
For use on progressing cavity positive displacement motors, two axially separated couplers are used to provide two points of flexure to permit the motor rotor centerline to orbit an extended centerline of the output shaft being driven.
The closed volumes of the sealed coupling are fluidly connected to variable volume chambers having an ambient fluid reference side to provide hydrostatic compensation and to provide make-up fluid to replace any leakage from the coupling seals. Axially moving sealed pistons are preferred. A sliding piston separating oil and mud, movable along a bore may comprise a hydrostatic compensator. The hydrostatic compensator tends to equalize pressure inside a sealed enclosure and the ambient pressure outside. Such compensators are widely used in sealed, drilling related, assemblies used in wells. They are well understood by those skilled in the art of drilling related machine design.
An optional ball mounting feature uses the ball to limit axial separation of the coupling to supplement the ability of the sleeve to prevent coupling separation.
It is an object of the invention to provide a stable point of flexure to provide radial stability to the seal to reliably seal the lubricant containing feature of the coupling.
It is another object of the invention to provide means to prevent separation of the coupling by means intrinsic to positioning and confining the sealing element.
It is yet another object to provide lubricant reservoir means to feed lubricant to the rubbing surfaces of the coupling and to provide hydrostatic compensation for sealed volumes in the coupling.
These and other objects, advantages, and features of this invention will be apparent to those skilled in the art from a consideration of this specification, including the attached claims and appended drawings